The present disclosure relates to the field of measuring the verticality of the movement of a drilling rig.
Measuring the verticality of a drilling rig conventionally consists in continuously or periodically measuring any departures from the vertical of a drilling rig that is intended in particular to cut trenches of great depth and small width, typically equal to the width of the drilling rig.
Such rigs are particularly, but not exclusively, rotary drum drilling rigs. They are described in particular in Document FR 2 211 027. The drilling rig is suspended from a hoist by means of cables. The rig continues to move downwards progressively as the rotary drums cut the trench.
For certain kinds of work, the trench may present great depth, which may be as much as one hundred meters or more. It is generally necessary for the trench to present great accuracy concerning its verticality, in particular because the final trench is usually the result of juxtaposing vertical boreholes. The accuracy that is required may be of millimeter order in the horizontal direction for each meter of depth of the trench.
In particular because of irregularities of the terrain in which the trench is to be made, there exist major risks of the drilling rig deviating from its vertical path, with this risk increasing as the depth of the borehole increases.
There therefore exists a real need to have systems that make it possible to monitor the verticality of the movements of the machine, by detecting any departures from the desired vertical path.
To solve this problem, Document EP 0 841 465 proposes a device for continuously measuring the verticality of a drilling rig, which device also makes it possible to detect twisting movements or crabwise movements of the rig.
The drilling rig described in that document thus includes a device for measuring the verticality of the movement of a drilling rig for cutting a vertical trench from the surface of the ground, said rig comprising a top end secured to means for supporting the rig while it is being lowered, a bottom end, and a vertical longitudinal axis,
the device further comprising:                two small-section cables, each cable having a first end fastened to the top end of the rig, the two fastening points being different;        means for keeping said cables tensioned;        two guide means arranged above the surface of the ground to ensure, at least at the measurement instants, that the portion of each cable that is arranged in a horizontal plane is kept stationary in said plane while the rig is being lowered, said guide means defining two fixed reference points in a spatial relationship with said fastener points when they are in said horizontal plane;        means for measuring in two mutually orthogonal horizontal directions the angles of inclination of the portion of each cable extending between said fastener points and said guide means;        means for measuring the lengths of the portions of cable extending between the fastener points and the guide means while the rig is being lowered;        means secured to the rig to measure the angles of inclination of the longitudinal axis of said rig in two mutually orthogonal horizontal directions; and        processor means for using said measured lengths and said measured angles of inclination to calculate the movements of the two cable fastener points in said orthogonal directions and for using the calculated movements of the fastener points of the two cables and the two measured angles of inclination of the vertical axis of the rig to calculate the movements of two different fixed points of the bottom end of said rig.        
It can be understood that by continuously measuring the length of the cables between the top end of the rig and the fixed guide means, and by measuring angles of inclination at the ends of the cables fastened to the rig, it is possible to calculate continuously the coordinates of the two fastener points of the cables and thus to calculate any departures in position of the cables from the reference positions defined using the two fixed guide means. In that document, provision is made for an inclinometer to be arranged between one end of the cable and a swivel system fastened to the rig.